This invention relates to a method for lining tubular structures such as pipe, particularly, but not exclusively, long lengths of pipe in situ such as underground gas or water and sewer mains.
Metal pipes may be lined with thermoplastic polymers to convey corrosive material such as acids, or the lining may be used simply to rehabilitate or prolong the life of old, existing water or gas supply pipes or other pipes which may carry relatively non-corrosive fluids.
It has been proposed in U.S. Pat. No. 3,429,954 to line metal pipe by inserting into a pipe a longitudinally oriented heat shrinkable, irradiated thermoplastic polymer tubing, the tubing having in its unoriented state an outer diameter greater than the internal diameter of the metal pipe but slightly smaller than the internal diameter when oriented, and heating the tubing to a temperature above the crystalline melting point of the polymer to longitudinally shrink the polymer tubing and cause it to expand radially and fit tightly against the internal wall of the pipe.
The polymer tubings employed in U.S. Pat. No. 3,429,954 were irradiated to cross-link the polymer. This technique may be satisfactory for small diameter pipe, which is normally obtained and installed from a large coil. However, polymeric pipe having a diameter of greater than 3 or 4 inches is generally available only in straight lengths of several feet each, which must be joined by butt fusion or butt solvent welding techniques. Cross-linked polymers cannot with confidence be joined by fusion or solvent welding and are inferior in this respect to non-crosslinked thermoplastic polymers.
In the U.S. patent it is also suggested that the same polymer liners be expanded by heating using one of three methods.
(a) application of hot air internally PA0 (b) passing the polymer lined pipe through a furnace so as to apply heat externally PA0 (c) immersing the polymer lined pipe in a propylene glycol bath.
While methods (b) and (c) for obvious reasons are impractical for the relining of long, continuous lengths of underground pipe, method (a) is not effective either because in long pipe sections the air or any other gas or fluid will cool rapidly as it progresses along the pipe and there will be insufficient heat to raise the entire length of the polymer liners to the necessary temperatures.
A further problem in the lining of long lengths of metal pipe is that to achieve the proper expansion of the polymer liners inside the metal pipe, the liner must be completely free to shrink in the longitudinal direction. If the shrinkage is prevented or only slightly hindered in any way, it will result in no or only a reduced and insufficient expansion, as well as undesirable axial tensile stresses in the pipe that may be harmful to the fused or solvent welded joint.
In particular, where long, continuous lengths of pipe are to be lined, problems arise where the weight and associated friction drag of the long lengths of the lining itself, inside the metal pipe tends to prevent the required shrinkage. This situation may be further aggravated where the metal pipe is not completely straight and the liner becomes pinched or where the polymer liner during the expansion process itself expands unevenly or at a different rate either at various points along the length of the liner where it may tend to hang up, or around the periphery of the liner where this may cause the liner to curve in the length direction inside the metal pipe and thereby become pinched.
It is the object of this invention to provide a method particularly for lining long lengths of underground pipe that will overcome all of the above mentioned difficulties and problems.